In certain mass spectrometers, an electron multiplier detects an incoming beam of positive ions. The multiplier consists of a hollow glass tube coated on the inside with semiconductive material. When the ions impinge on the coating, they are converted into electrons, and the number of electrons is amplified by successive further collisions with the tube wall. The amplified number of electrons reaching the end of the tube generates a current at the output lead.
To reduce noise levels in the multiplier output caused by stray electrons and photons entering along with the positive ions, it is known to transversely displace the multiplier inlet from the trajectory of entering particles. The ions enter through an aperture in a positively-charged (e.g., +100 volts) plate. Beyond the plate is a deflector, electrically connected to the plate and thus also positively charged, and the multiplier inlet, which carries a high negative voltage (e.g., 1000 to 3500 volts). The ions are deflected toward the multiplier inlet by the electric field generated between the multiplier inlet and the plate and deflector. One such detector is the Galileo Channeltron Electron Multiplier Model 4772.
The same deflector has been used as a Faraday cup to measure the unamplified number of ions, such as for measuring the gain of the multiplier and for detecting unusually strong beams that would saturate the multiplier. In the Faraday-cup mode, the positive charge on the deflector and plate is removed to permit the ions to impinge thereon, and the current thereby generated is measured. The switchover between modes has conventionally required that a high-positive-voltage lead be removed from the deflector and plate and a current-measuring lead be substituted.